Conventional electromagnetic valve actuators comprise both resilient displacement means and electromagnetic actuator means for actuating the moving member between two extreme positions which correspond respectively to the open position of the valve and to the closed position of the valve. The resilient displacement means generally comprise a spring associated with the moving member to return it elastically into its extreme open position, and a spring associated with the valve stem in order to return it elastically into its closed position so as to urge the moving member elastically into its extreme closed position. The electromagnetic actuator means generally comprise an electromagnetic displacement means for bringing and/or holding the moving member in its extreme open position, and an electromagnet for bringing and/or holding the moving member in its extreme closed position.
In order to be certain that the valve is properly pressed against its seat when the moving member is in its extreme closed position and therefore the moving member and the valve are not connected to each other, timing clearance is provided between the moving member and the valve stem when the moving member is in its extreme closed position and the valve is properly pressed against its seat by the spring which is associated therewith.
Under such circumstances, if the moving member is brought quickly from the extreme closed position to the extreme open position, the moving member will strike the valve stem. This impact, which in any event is noisy, leads to mechanical stresses and wear of the moving member and of the valve stem that can, in extreme cases, lead to damage thereof. It is therefore important to know precisely the amount of timing clearance that exists so that the electromagnetic means can be controlled firstly to bring the moving member gently into contact with the valve stem, and then to accelerate the moving member once it is pressing against the valve stem.
Unfortunately, timing clearance varies while the engine is in operation, in particular as a function of temperature, and also over the lifetime of the engine, in particular as a function of the wear of the valve and of the various components of the valve actuator. In addition, timing clearance can vary from one valve actuator to another as a function of their manufacturing tolerances.
In order to determine the timing clearance, a conventional method comprises controlling the valve actuator means to move the moving member from the extreme closed position to the extreme open position and to detect variation in an electrical characteristic of the electromagnetic actuator means. This variation in the electrical characteristic is caused by the increase in the opposition to movement of the moving member once it encounters the valve stem. The timing clearance is then determined by measuring the time that elapses between the beginning of the moving member being set into motion and the start of variation in the electrical characteristic. Nevertheless, determining timing clearance in that way turns out to be relatively inaccurate. Thus, there is a need for a method of determining timing clearance simply and accurately.